The core vocabulary of Esperanto was defined by Lingvo internacia , published by Zamenhof in 1887. It comprised 900 roots, which could be expanded into tens of thousands of words with prefixes, suffixes, and compounding. In 1894, Zamenhof published the first Esperanto dictionary, Universala Vortaro , with a larger set of roots. However, the rules of the language allowed speakers to borrow new roots as needed, recommending only that they look for the most international forms, and then derive related meanings from these.
Since then, many words have been borrowed, primarily but not solely from the Western European languages. Not all proposed borrowings catch on, but many do, especially technical and scientific terms. Terms for everyday use, on the other hand, are more likely to be derived from existing roots—for example komputilo (a computer) from komputi (to compute) plus the suffix -ilo (tool)—or to be covered by extending the meanings of existing words (for example muso (a mouse), as in English, now also means a computer input device). There are frequent debates among Esperanto speakers about whether a particular borrowing is justified or whether the need can be met by deriving from or extending the meaning of existing words.
In addition to the root words and the rules for combining them, a learner of Esperanto must memorize some idiomatic compounds that are not entirely straightforward. For example, eldoni , literally "to give out", is used for "to publish" (a calque of words in several European languages with the same derivation), and vortaro , literally "a collection of words", means "a glossary" or "a dictionary". Such forms are modeled after usage in some European languages, and speakers of other languages may find them illogical. Fossilized derivations inherited from Esperanto's source languages may be similarly obscure, such as the opaque connection the root word centralo "power station" has with centro "center". Compounds with -um- are overtly arbitrary, and must be learned individually, as -um- has no defined meaning. It turns dekstren "to the right" into dekstrumen "clockwise", and komuna "common/shared" into komunumo "community", for example.
Nevertheless, there are not nearly as many idiomatic or slang words in Esperanto as in ethnic languages, as these tend to make international communication difficult, working against Esperanto's main goal.
Here are some useful Esperanto phrases, with IPA transcriptions: